A recent trend in automobile drive technology, as part of developments in the automobile electronics sector, is for established passive safety systems like seatbelts and airbags to be extended by active safety systems, such as anti-lock braking systems (ABS), electronic stability programs (ESP) and electrical steering systems, to provide an increasing range of driver assistance functionalities. As has already been the case in the drive train for some time, system complexity is also continuously increasing here in order to detect hazardous driving situations and contribute to accident avoidance through active interventions by a control system. With ongoing technological advances, these trends are expected to continue and grow stronger in the future.
The resulting significant increase in the number of electronic components with a safety-related functionality has given rise to previously unprecedented requirements in terms of reliability and system availability. In order to be able to achieve this while at the same time meeting cost objectives, it is desired to develop efficient methods for functional monitoring through integrated test methods along with designed redundancies. At the same time, progress is desired in design methodologies in order to be able to identify and avoid possible weaknesses in safety systems early on. In the area of magnetic field sensors, for example, this has been done by the introduction of the Safety Integrity Level (SIL) standard.
In order to meet SIL standards in the automotive field, it is desired to implement and use corresponding self-tests, including built-in tests, not only at start-up but also during normal operation, as well as automatic monitoring structures or corresponding redundant functional blocks and/or signal paths. Conventional magnetic sensor systems, such Hall-effect or magnetoresistive (xMR) systems, often have used a single-channel analog main signal path, such as the one depicted as an example in FIG. 1A in which the analog signals from a Hall sensor 10, measuring a magnetic property of a mechanical system 12, are passed through a single-channel path and via analog-to-digital converter (ADC) 14. It is technically very difficult, or perhaps even impossible, to meet the SIL requirements in safety-critical applications with this concept and therefore it is usually not possible to cover safety requirements with a single feed-forward sensor system.
Thus, other conventional solutions, such as the one depicted in FIG. 1B, have used two redundant magnetic field sensors 40 and 42 to meet SIL requirements. Obviously, a considerable drawback of these solutions is the corresponding doubling of the cost, for not one but two sensors. Still other solutions, such as one depicted in FIG. 1C, propose a defined superimposed test signal 70 outside the characteristic signal frequency ranges of the system, such as magnetic field sensors with an additional on-chip conductor loop or pressure sensors with superimposed electrostatic coupling to the sensor. Obviously, such test signals could be also generated outside the sensor system, e.g., by a device connected to the mechanical system 72 influencing the magnetic measurement and thus providing feedback which can be diagnosed. Nevertheless, such setups can be more expensive, may require a higher power consumption, and may introduce influences to the main function as well and require a specific physical set-up which is not feasible in many cases, which also means they cannot be used in general.
Additionally, conventional approaches do not address or provide self-testing and related diagnostic coverage of two-dimensional (2D) or three-dimensional (3D) sensors, which are increasingly used in automotive and other high-integrity applications. For example, 3D sensors can be used in angular speed applications, such as off-axis angle and angle speed measurements in brushless DC motors or steering angle sensors, among others. The addition of a third axis can provide safety advantages using additional attributes form the related mechanical system, though sufficient diagnostic coverage of the sensor itself remains an important task. Fundamentally, implementing highly integrated and thus cost-effective diagnostic functionality for sensors within a given electromechanical system in safety-critical applications, such as those required to meet SIL and/or other applicable safety standards, remains a challenge.